1. Field of the Invention
The invention relates to a rack assist type electric power steering system.
2. Description of Related Art
In a conventional rack assist type electric power steering system, since the coefficient of expansion of a rack shaft is different from the coefficient of expansion of a rack housing that accommodates the rack shaft, the rack shaft moves in a shaft length direction relative to the rack housing due to a change in an ambient temperature. Therefore, a detected rotation angle of a steering shaft is deviated with respect to the actual rotation angle by an amount corresponding to the amount of thermal displacement of the steering shaft. Therefore, in an electric power steering system described in, for example, Japanese Patent Application Publication No. 2010-30469 (JP 2010-30469 A), an ambient temperature around a steering mechanism is detected by a temperature sensor and the detected temperature is provided to a steering angle calculation portion. When the steering angle calculation portion calculates a steering angle by using the combination of a detection value obtained by a shaft angle detector and a detection value obtained by a motor angle detector that detects a rotation angle of a motor for steering angle assistance, the steering angle calculation portion corrects the detection value obtained by the shaft angle detector or the motor angle detector on the basis of a temperature detected by the temperature sensor and calculates the steering angle by using the corrected detection value.
In the system described in Japanese Patent Application Publication No. 2010-30469 (JP 2010-30469 A), correction is performed to cancel an amount of deviation between the detection value obtained by the shaft angle detector or the motor angle detector and the actual steering angle at the time when the ambient temperature around the rack shaft has changed. However, this correction is correction for correcting the number of turns of the steering shaft, that is, the number of revolutions (±1 revolution, ±2 revolutions, or the like) based on a steering angle middle point of the steering shaft, at the time when an initial steering angle is calculated. This correction is not correction for correcting the detection value obtained by the shaft angle detector or the motor angle detector during one rotation of the steering shaft. Therefore, the detection value obtained by the shaft angle detector or the motor angle detector still includes a deviation. Accordingly, the steering angle that is calculated on the basis of the detection value obtained by the shaft angle detector or the motor angle detector also includes a deviation.